In the past, planographic printing plates like PS plates had been contacted with a sheet referred to as a slip sheet and wound into the shape of a coil or cut to a desired size and deposited to protect an image-forming surface layer (image-forming surface) by performing surface treatment such as pebbling, anodic oxidation, silicate treatment or other chemical treatment on a support such as an aluminum plate either formed into the shape a coil or a sheet independently or in a suitable combination followed by coating an image-forming coating material on the surface to form an image-forming layer. A method in which the slip sheet is charged was typically employed in the process for adhering this slip sheet to the image-forming surface of a planographic printing plate.
The amount of the charge for charging the slip sheet is required to be a suitable value for satisfactory paper discharging characteristics in a plate setter and so forth that contains an automatic plate feeder. For example, if the amount of the charge is too small, problems in the form of the occurrence of misalignment and separation occur in the image-forming surface in the production process of the planographic printing plate. In addition, if the charge is too large, due to the excessively high adhesion between the slip sheet and the image-forming surface, problems occur in the slip sheet separation and paper discharge steps in a plate setter that contains an automatic plate feeder as previously mentioned.
FIGS. 1A and 1B are schematic drawings of the production process of a planographic printing plate. FIG. 1A is a drawing of a step in which the slip sheet is adhered and cut, while FIG. 1B is an enlarged schematic drawing of a laminate of a layered planographic printing plate following cutting. In FIGS. 1A and 1B, reference symbol (1) indicates the slip sheet, (2) indicates an image-forming surface of a planographic printing plate, (3) indicates a support surface of a planographic printing plate, (4) indicates the surface of the slip sheet, (5) indicates a charging device, (5′) indicates a charging roll, (6) indicates a rotary cutter, (7) indicates a transport belt conveyor, (8) indicates a laminate of the planographic printing plate and slip sheet, (9) indicates a resin pallet, and (10 indicates a planographic printing plate.
FIG. 2 is shown a schematic drawing of an automatic plate feeder for planographic printing plates. In FIG. 2, reference symbol (11) indicates a suction member for planographic printing plates, (13) indicates a suction member for slip sheet discharge, (14) indicates rolls for slip sheet discharge, (15) indicates a slip sheet discharge storage box, and (16) indicates the stored slip sheets, while other reference symbols have the same meanings as those indicated in FIG. 1A or 1B.
A process for preventing the aforementioned problems by using a slip sheet in which the volume resistivity value is adjusted to a certain value is disclosed in Japanese Unexamined Patent Application, First Publication No. 10-197992 as a countermeasure for these problems. However, in the case of fluctuations in the line speed or environment of the production process, it is difficult to control the amount of charge to a constant amount, thereby making this inadequate. In addition, although a process for controlling the amount of charge to a constant level by measuring the surface electrical potential of a slip sheet is disclosed in Japanese Unexamined Patent Application, First Publication No. 2001-22511, this process had the problems of complex production equipment and high costs.
In addition, in recent platemaking processes for planographic printing plates such as PS plates, the flow of the platemaking work is being reconfigured from platemaking processes involving adhesion and exposure using a film as in the prior art to the sophisticated digital processing of data and the development of high-output lasers for recording that data, and the parallel development of accompanying intensifiers, resulting in the rapid proliferation of computer-to-plate (CTP) systems in which a camera-ready copy produced with a computer is exposed and developed directly on the planographic printing plate without having to go through the stage of a film copy. Accompanying these technological advances, automation of the platemaking process has proceeded rapidly for the purpose of improving the efficiency of platemaking work. A water-soluble oxygen-blocking layer is formed in these CTP-based planographic printing plates, and in planographic printing plates accompanied by a laser-reactive heat-sensitive layer on the image-forming surface in particular. When the number of laminated layers is increased in the production process since this layer is soft and since the surface is softened by moisture and so forth, a blocking phenomenon occurs between the image-forming surface of slip sheet, thereby resulting in the problem of increased susceptibility to sticking.
Moreover, in planographic printing plate accompanied by a heat-sensitive layer as described above, as is disclosed in Japanese Unexamined Patent Application, First Publication No. 10-324734 and Japanese Unexamined International Patent Publication No. 2001-520953, heat treatment (aging) may be performed after coating and drying an image-forming coating material onto a support, and at this time, the image-forming surface softens due to the temperature of the heat treatment, thereby resulting in the surface of the slip sheet sticking to the image-forming surface and damage to the surface of the image-forming surface when the slip sheet is separated. Although a slip sheet having satisfactory separation has been used to solve this problem, in which polyethylene is laminated to the surface of the slip sheet that contacts the image-forming surface of the planographic printing plate, the surface of this polyethylene layer is easily charged, and has the property of difficulty in dissipating the charge once it has become charged, thereby resulting in problems with separation of the slip sheet in a plate setter and so forth that contains an automatic plate feeder as previously mentioned.
Moreover, although the slip sheet discharged with a plotter is temporarily housed in a box for discarding, with respect to the storage characteristics of the slip paper in the discard box, it is preferable that the slip sheet be housed in the box while being folded in an orderly manner to facilitate efficient storage.